This invention relates to the production of cuprous oxide from cupric oxide or mixtures of cupric oxide and minor amounts of cuprous oxide and/or metallic copper. The cuprous oxide product is of sufficient purity to be used as a pigment.
Cuprous oxide, also known as red copper oxide, finds extensive use as a pigment in antifouling paints, as an agricultural fungicide, as a porcelain glaze and in a variety of other uses. There are a number of known ways to manufacture cuprous oxide. It can be produced by the high temperature air oxidation of metallic copper, by the addition of bases to aqueous solutions of cuprous salts such as cuprous chloride, by the action of glucose on cupric hydroxide and by the selective reduction of cupric oxide.
Substantial quantities of mixed copper oxides are produced as byproducts of copper refining and fabrication, as scale from the anodes of copper refineries, from certain wrought copper products after annealing and from drawing wire. Use of this byproduct as the raw material for production of highgrade cuprous oxide offers substantial economic advantages as compared to the use of other copper-containing source materials.
It has been proposed to produce cuprous oxide by the reduction of cupric oxide using carbon as a solid reducing agent. This process is illustrated by the Ayers patent, U.S. Pat. NO. 2,554,319. Ayers teaches the mixing of finely-divided cupric oxide and carbon in stoichiometric proportions and passing the mixture through a heating zone at a temperature of about 1400.degree. C whereby reduction of the cupric oxide is accomplished. Carbon is oxidized to carbon monoxide. Finely-divided carbon is necessary in Ayers' process as lumps or aggregates of carbon cause reduction of cupric oxide adjacent the carbon lumps to metallic copper.
It is also known from thermodynamic data that cupric oxide will thermally decompose to cuprous oxide and oxygen at relatively high temperatures. For example, the partial pressure of oxygen inequilibrium with cupric and cuprous oxide at 1200.degree. K is approximately 0.025 atm. Thus, the decomposition of cupric oxide at 1200.degree. K should proceed as long as the oxygen pressure in the reaction environment is kept well below 0.025 atm. This can be done in theory by flowing an inert gas through the reaction zone to flush the oxygen product or by carrying out the decomposition in vacuum. In practice, these procedures require higher temperatures to attain practical reaction rates but with cupric oxide, maximum temperature is limited to about 950.degree. C, or about 1223.degree. K, as sintering begins at about that temperature.